The ethylene stream from the overhead of a deethanizer unit in a naphta cracker contains about 0.5 to 2.0% by weight of acetylene which is a poison in the ensuing process of ethylene polymerisation, so that the ethylene impurities should be lowered to below 5 ppm. Selective hydrogenation is a common method to remove acetylene in the stream. Two factors are important in this process. One is the ethylene selectivity, i.e. the fraction of acetylene produced by acetylene conversion, and the other is the catalyst lifetime which is limited by green-oil deposition during reaction.
For the selective hydrogenation of acetylene to ethylene, noble metal supported catalysts are usually used and especially, palladium-based catalyst is known to show high activity and high ethylene selectivity.
According to Bond et al. the ethylene selectivity on transition metals is lowered in the sequence of Pd>Rh, Pt>Ni>>Ir (“Catalysis by metals”, Academic Press, New York, 281-309, 1962).
A catalyst preparation method for impregnating palladium on silica is presented in U.S. Pat. No. 4,387,258 and a catalyst preparation method of palladium/titania is presented in U.S. Pat. No. 4,829,329.
Besides silica and titania, alumina is also commercially used as a support for catalysts used in acetylene hydrogenation. These supported catalysts are easily deactivated by the formation of green oil, which is a side reaction occurring on the support surface. This green oil blocks the pores and covers the active sites, so that the phenomenon shortens the regeneration cycle and catalyst lifetime.
In the acetylene hydrogenation, ethylene selectivity as well as catalyst lifetime is important.
According to Bond and Well, the reason for why acetylene is selectively hydrogenated, despite the fact that the hydrogenation rate of ethylene is faster by 10 to 100 times than that of acetylene, is because the adsorption strength of acetylene is much stronger than that of ethylene. Hence acetylene hydrogenation is dominant when there is a competition of hydrogenation between acetylene and ethylene. So the reaction on the catalyst is determined largely by the rate of adsorption and desorption instead of the rate of surface reaction. According to the analysis of the property of the 8B transition metals, including Pd, for the adsorption of acetylene, ethylene or propylene, the rate of adsorption decreased in the following order and the rate of desorption decreased in the reverse order. Acetylene>Diolefin>Olefin>Paraffin (The Oil and Gas Journal, 27, 66 (1972)).
Therefore, if we add diolefin as an additive to the reactant stream of acetylene hydrogenation, we can suppress the adsorption of ethylene and consequently, can selectively hydrogenate acetylene to ethylene. This diolefin, which has adsorption strength stronger than that of ethylene and weaker than that of acetylene, is called as a moderator. But the diolefin itself induces the green oil formation, and furthermore, a separation of unreacted diolefin after acetylene hydrogenation is difficult. For this reason, carbon monoxide, which also acts as a moderator in acetylene hydrogenation, is preferred.
A method to increase ethylene selectivity by carbon monoxide is presented in U.S. Pat. No. 3,325,556 and No. 4,906,800. But, carbon monoxide also enhances the formation of green oil by carbonylation reaction, thus, the problem of catalyst regeneration cycle and catalyst lifetime still exists.
Titanium promoter was proposed as an additive to solve the catalyst deactivation problem in acetylene hydrogenation, and the detail is presented in Korean patent No. 2000-0059743. When palladium catalyst is modified with titanium species and reduced at high temperatures, such as 500° C., titanium oxide species is partially reduced and migrates onto the Pd surface and electron is transferred from titanium oxide to palladium, making palladium an electron rich surface. This is called a Strong Metal-Support Interaction (SMSI). The SMSI phenomenon increases the ethylene selectivity, and retards the catalyst deactivation. But, the highest ethylene selectivity of Pd—Ti-catalyst in the experimental condition described in the patent is about 90%, which still needs further improvement.
For the Strong Metal-Support interaction between Ti and Pd to occur, high temperature reduction, such as 500° C., is necessary, but the maximum temperature, which can be elevated inside the industrial reactor, is about 300° C., therefore, the improvement of ethylene selectivity by titanium is limited in the industrial process.